The objective of the proposed research is to visualize GABAergic neurons, terminals, and receptor sites m medial vestibular nucleus (MVN) of normal and surgically-manipulated animals in order to elucidate the anatomical basis for GABAergic control of this nucleus. This is basic information which is essential for understanding the organization of the vestibulo-ocular reflex (VOR). In addition, identification and characterization of baclofen-sensitive GABA-B binding sites in NM should lead to an understanding of how inhibition is mediated in neurons which are responsible for velocity storage, one component of the VOR. This objective will be accomplished using an immunocytochemical approach to specify the neurotransmitter-defined neuronal circuits at the light microscopic level, and to characterize and quantify the synaptic arrangements visible ultrastructurally. The studies will be based on single and double label immunocytochemical techniques employing anti-GABA antibodies to identify GABAergic elements, and antibodies directed against the p-chlorophenyl moiety of L-baclofen, a specific GABA-B receptor agonist, to identify baclofen-sensitive GABA-B binding sites in animals pretreated with the drug. The first specific aim is to localize and characterize the GABA and/or baclofen-immunoreactive cells and terminals present at different rostrocaudal levels of MVN of normal cynomolgus monkeys. The second specific aim is to examine immunocytochemically-reacted tissue from animals who have lost velocity storage due to midline medullary section of the vestibular commissural fibers. These studies will allow visualization of GABAergic interactions involving MVN neurons that participate in velocity storage. The third specific aim is to use nodulo-uvulectomized animals to examine degenerating GABAergic Purkinje cell afferents at various levels of MVN. The nodulus and ventral uvula exert inhibitory control of velocity storage, acting through GABA-B receptors. The fourth specific aim is to compare degeneration in MVN after nodulo-uvulectomy to that following ablation of the flocculus and paraflocculus. This lesion has no effect on velocity storage, but causes a loss of visual suppression and adaptation of VOR gain in response to visual stimuli. The proposed research will define the distribution and morphologic characteristics of the GABAergic neurons of MVN, including those with commissural axons, and those which receive afferents from the nodulus and ventral uvula, and the flocculus and paraflocculus. The studies will provide the first direct demonstration of the cellular distribution and subcellular localization of L-baclofen-sensitive GABA-B binding sites in normal and experimental monkey vestibular nuclear tissue. Combining morphological and physiological techniques, we expect to provide critical information about the alterations in synaptology that accompany specific manipulations of the central vestibular system. As such, these studies should help to specify the role of GABAergic inhibition in the VOR, in the organization of the velocity storage mechanism, and in the modulations in synaptology that underlie vestibular habituation and adaptation.